Re: [Fwd: [RFT] major libata update]

[Jeff Garzik <jeff@garzik.org>]

Linus Torvalds wrote:
> 
> On Wed, 17 May 2006, James Bottomley wrote:
>> This is one of the questions.  Currently block has no concept of "host".
> 
> That's good. 
> 
> I don't understand why you'd ever _want_ a concept of "host". The whole 
> concept is broken and unnecessary. At no point should you even need to map 
> from request to host, but if you do, you don't need to introduce any 
> generic "host" notion, you can do it easily per-queue.

"host" is ultimately the wrong word, sure.

It's more about (a) grouping queues into a topology that the user 
expects, as exported by sysfs, and (b) grouping queues for the purposes 
of useful and/or necessary hardware operations like "stop <these> 
queues, so that we can bitbang the hardware".

That grouping of queues, along with the lib-ification of highly common 
request management code[1], is part of the non-SCSI utility that libata 
derives from drivers/scsi.

"group-wide operations" are highly common, and generic code inevitably 
results from that.  But IMO that's helper code, living on top of the 
perfectly-fine existing code.


> The whole fixation with "host" in the SCSI layer is a bug, I think. What 
> does it matter, really? And when do you actually have a "request_queue" 
> entry without already knowing which controller it is connected to (ie why 
> do you even need that mapping)?

True, the mapping is obtained from request_queue not request.

The entry point into a block driver is via q->request_fn(), which only 
has a request_queue for an argument.  So in practice, one usually 
obtains the private controller-info and bus-info data via the 
request_queue's ->queuedata.

Deep into sub-APIs, I've seen that sometimes you'll see only the request 
passed as an argument, because it's easier to walk 
request->queue->controller_info than to pass additional arguments to 
every function.

	Jeff


[1] "resource management":  refers to drivers/scsi's handling of 'device 
busy', 'group-of-queues busy' style transient errors, well integrated 
with block layer's command queueing and well synchronized with the EH 
thread.